The present invention relates to digital images and, more particularly, to a method of scrambling the images of a digital video sequence that is tolerant of transmission induced data errors.
Conditional access refers to the scrambling or encryption of a signal so that only a user having access to a key can gain access to the original signal. Conditional access permits a provider to supply services only to those users authorized to receive the service and is important to pay television, video-on-demand services, real-time video multi-cast services, and security systems. Conditional access is used in conjunction with video signals transmitted over a variety of communication channels including wireless broadcast, satellite communication, home networks, and the Internet.
Signal scrambling or encryption required for conditional access is problematic when used in conjunction with digital video. Typically, digital video includes a compression process. Without compression, the quantity of data necessary to digitally describe the succession of images making up a video sequence would be so great that transmission or storage would be impractical for many uses. On the other hand, compression produces interdependent data and data losses and errors resulting from transmission can cause an error to propagate to future images of the sequence.
The prior art of video scrambling includes methods that scramble a video signal in the spatial domain. For example, Hobbs, U.S. Pat. No. 5,815,572 discloses a number of techniques for scrambling the scan line signals of NTSC (National Television Standards Committee) analog television. One scrambling technique reverses the scan line signal so that the right end of the line becomes the left end and vice versa. Another disclosed scrambling method permutes the order of lines in a block comprising a plurality of scan lines. Similarly, the order of a plurality of blocks of lines making up a field or image can be permuted. In addition, a line might be scrambled by inverting the line signal. For instance, if a higher value of a luminance signal in a scan line represents BLACK and a lower value represents WHITE, the values could be inverted so that the lower values represent BLACK and the higher values represent WHITE. Generally, scrambling signals in the spatial domain significantly affects the statistical properties of the signal making it difficult to apply compression to reduce redundancies in the video signals. In addition, the correlation of video data that is scrambled in the spatial domain can be used as a basis for unscrambling the signal and obtaining unauthorized access to the video.
A second general method of scrambling digital images treats compressed image data as ordinary data to be encrypted with traditional cryptographic methods. Pinder et al., U.S. Pat. No. 5,684,876, disclose a method of applying a block cipher to the payload of MPEG Transport Stream packets. The disclosure indicates that a preferred cipher is the Digital Encryption Standard (DES). Likewise, the Real-Time Transport Protocol (RTP) which is commonly used in conjunction with the User Datagram Protocol (UDP) for digital video transmission supports encryption of packet payload data. However, considerable processing overhead is required to encrypt data at the high rates required for real time video. On the other hand, multimedia data may not be of sufficient value to justify the high cost of data encryption. To reduce the processing overhead, encryption can be applied to selected frames (for example, intra-coded frames) however, such limited encryption may not be sufficiently secure for many applications. The presence of known data such as synchronization and end of block symbols in the MPEG Transport Stream provides a basis for unscrambling the data and obtaining unauthorized access. Further, in some applications it is desirable to transcode the data so that the data may be transmitted over a communication channel requiring a data rate different from the data rate of the original encoding. The scrambling key must be available at transcoding points to permit decryption, decompression, re-compression and re-encryption of the data. Making the scrambling key available at a number of transcoding points is inconvenient and can jeopardize the security of the encryption.
One of the inventors of the present invention has proposed a method of video scrambling in which image data is shuffled following transformation. As described by Zeng et al. in the paper EFFICIENT FREQUENCY DOMAIN VIDEO SCRAMBLING FOR CONTENT ACCESS CONTROL, Proceedings of ACM Multimedia, November 1999, digital video may be scrambled by shuffling transform coefficients among the macroblocks making up a horizontal slice of a constituent image. Several scrambling techniques are disclosed providing excellent security for the signal. Since the scrambling is applied to the transformed image data, the statistical properties of the original image and the efficiency of the compression process are essentially unaffected by scrambling. However, video data is often transmitted over noisy communication channels by protocols providing unreliable delivery. As a result, data may be lost during transmission and retransmission or error correction is often impractical. Since packetization of video data proceeds along the horizontal slices of the image, video is susceptible to packet loss. Scrambling data along the direction of packetization may leave the image vulnerable to data errors because of the interdependence of the scrambled data describing the slice.
What is desired, therefore, is a method of image scrambling that provides appropriate security, utilizes reasonable data processing resources, has minimal impact on the efficiency of the compression process, and enables fault tolerant handling of errors or losses in the transmitted data.